The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with single photon emission computed tomography (SPECT) scanners with fan-beam collimation for medical diagnostic imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will have other applications in which fan-beam type data is reconstructed into an image representation for medical, quality assurance, and other examinations.
In single photon emission computed tomography (SPECT), the resolution of the collimated detector deteriorates with increased distance from the face of the collimator. Thus, it is desirable to place the gamma camera as close as possible to the patient to reduce the blurring caused by the distance-dependent system response function and to minimize loss of resolution. To accomplish this, it has been common practice to use non-circular orbits in which the detectors follow the body contour but with the central projection ray of each detector always passing through the center-of-rotation. For parallel collimation, this simplifies the reconstruction algorithm and maintains full body viewing for all projection angles. However, the potential of truncating a selected region of interest increases when imaging with converging collimators.
When imaging off-center organs, such as the heart, particularly with converging collimators with the detector center ray through the center-of-rotation, the off-center organ shifts within the field-of-view and even move partially out of the field-of-view for some projection angles. Parts of the subject which move in and out of the field-of-view resulting in image truncation. Truncation artifacts of the organ of interest are reduced in reconstructed tomograms when a fan-beam detector follows an orbit where the central ray is not constrained to pass through the center-of-rotation.
The challenge in imaging the heart with converging collimators is that the heart is not centrally located in the body and is usually located off the center-of-rotation. It is difficult to position the heart centrally in the field-of-view because the detector is constrained by the outer contour of the body. By restricting the central projection ray to pass through the center-of-rotation, the likelihood of image truncation of the organ of interest greatly increases. The heart can be kept in the field-of-view for each projection angle either by translating or rotating the detector. A second approach may be easier to implement in practice. This is accomplished with fan-beam collimation and rotating the detector gantry in equal angular increments so that the ray from the center-of-rotation to the center of the detector rotates in equal angular increments and at each projection the center of the detector is rotated or translated to place off-center organs in the field-of-view.
A related backprojection algorithm is presented in "An Elliptical Orbit Backprojection Filtering Algorithm For SPECT", G. Gullberg and G. Zeng, 1992 IEEE Nuclear Science Symposium, presented October 1992, printed March 1993, for parallel-beam geometry.
The present invention provides a new and improved reconstruction algorithm for a fan-beam detector, which overcomes the above-referenced truncation problems and others.